A lightweight, high-strength, and durable material is required for the material of the bodywork (outer panel) of an aircraft. Therefore, in recent years, resin materials (composite materials) reinforced with fiber have been increasingly used.
As such composite materials, for example, carbon-fiber reinforced plastics (CFRP) in which carbon fibers are hardened with an epoxy resin etc. and glass-fiber reinforced plastics (GFRP) in which glass fibers are hardened with an epoxy resin etc. are often used.
These composite materials have a problem in that resistance to a lightning strike is lower than that of metal. Furthermore, since fasteners for mounting an outer panel to an inner structural member are made of metal (for example, titanium alloy), there is a risk of a lightning strike passing through the fasteners to generate, for example, a spark therein.
Therefore, when a composite material is used for the outer panel, a structure considering lightning protection, in particular, a structure for preventing lightning from passing through the fasteners, is required.
Examples of such a lightning protection structure are described in Patent Document 1 and Patent Document 2.
The structure shown in Patent Document 1 has an insulating cap at the outer end of the head of the fastener. There are many proposals for preventing passage of lightning by devising the structure of the fastener in this way.
The structure shown in Patent Document 2 has a metal strap (electrically conductive layer) around the head of the fastener and an insulating layer on the head to release a lightning strike to an outer panel surface.
Patent Document 1: U.S. Pat. No. 4,630,168
Patent Document 2: U.S. Pat. No. 5,845,872